This invention relates to an optical transmission method and apparatus suitable for use with a terminal station apparatus for an optical communication system for a very long distance of up to several thousands kilometers across an ocean as well as an optical amplification method and apparatus suitable for use with a repeater for an optical communication system.
A repeater for use with a very long distance transmission system across an ocean of up to several thousands kilometers is constructed as a regeneration repeater which converts signal light once into an electric signal, amplifies the electric signal, converts the electric signal back into signal light and then repeats the signal light. Now, investigations are directed to realization of an optical amplifier multi-stage repeating system wherein, replacing such regeneration repeater by an optical amplifier repeater which amplifies signal light, a plurality of optical amplifier repeaters are connected in a chain to effect optical communications.
Since the optical amplifier multi-stage repeating system has a function of optical amplification without relying upon the transmission rate, it is anticipated that the transmission rate is raised by improving the terminal station apparatus on the opposite sides to reduce the number of parts significantly, assure a high reliability and achieve reduction of the cost.
As an optical amplifier (repeater) for use with the optical amplifier multi-stage repeating system, investigations are directed to a doped fiber amplifier which is accommodated in a single repeater housing for each of an ascending line and a descending line and employs an excitation light source in an amplifier of each of the ascending line and the descending line. The doped fiber amplifier is constituted mainly from a doped fiber, an excitation light source, and an optical coupler (or wave combiner).
An exemplary one of popular doped fiber amplifiers is shown in FIG. 51. Referring to FIG. 51, the doped fiber amplifier shown includes an input port 101 for inputting signal light therethrough, an excitation light source 102 for outputting excitation light, a wave combiner 103 for coupling signal light from the input port 101 and excitation light from the excitation light source 102, a doped fiber 104 for amplifying light from the wave combiner 103, and an output port 105 for outputting light from the doped fiber 104.
The doped fiber amplifier shown in FIG. 51 has a so-called forward excitation construction wherein the wave combiner 103 is provided on the input side of the doped fiber 104 and excitation light from the excitation light source 102 is inputted to the doped fiber 104 from the front of the same.
In the popular doped fiber amplifier shown in FIG. 51 and having the construction just described, excitation light outputted from the excitation light source 102 is inputted to the doped fiber 104 using the wave combiner 103, and also signal light from the input port 101 is inputted to the doped fiber 104 using the wave combiner 103. In the doped fiber 104, the signal light is excited by the thus inputted excitation light in accordance with the energy level of doped ions of the doped fiber 104. Thus, the signal light is increased by stimulated emission of the excitation light with the signal light inputted from the input port 101.
FIG. 52 shows another doped fiber amplifier which has a so-called rearward excitation construction. The doped fiber amplifier shown in FIG. 52 is different from the doped fiber amplifier shown in FIG. 51 in that the wave combiner 103 is provided on the output side of the doped fiber 104 and excitation light from the excitation light source 102 is inputted from the rear of the doped fiber 104.
FIG. 53 shows a further doped fiber amplifier which has a so-called bidirectional excitation construction. Referring to FIG. 53, a first wave combiner 103a is provided on the input side of the doped fiber 104 and excitation light from an excitation light source 102a is inputted from the front of the doped fiber 104 while a second wave combiner 103b is provided on the output side of the doped fiber 104 and excitation light from another excitation light source 102b is inputted from the rear of the doped fiber 104.
It is to be noted that the input port 101, the doped fiber 104 and the output port 105 have similar functions to those of the doped fiber amplifiers described hereinabove with reference to FIGS. 51 and 52.
Meanwhile, various improvements to the amplifiers described above have been proposed wherein output light from a single excitation light source is supplied to both of an ascending line and a descending line from the point of view of reduction of the number of parts and enhancement of the reliability.
One of such improved doped fiber amplifiers is shown in FIG. 54. Referring to FIG. 54, the improved doped fiber amplifier shown has a forward excitation construction and includes an input port 201a for inputting signal light (first signal light) of an ascending line therethrough, a wave combiner 203a for coupling signal light of the ascending line and excitation light, a first doped fiber 204a for amplifying light from the wave combiner 203a, an output port 205a for outputting light from the first doped fiber 204a, an input port 201b for inputting signal light (second signal light) of a descending line, a wave combiner 203b for coupling signal light of the descending line and excitation light, a second doped fiber 204b for amplifying light from the wave combiner 203b, an output portion 205b for outputting light from the second doped fiber 204b, a first excitation light source 202a and a second excitation light source 202b for outputting excitation light, and a coupler 206 for splitting excitation light outputted from the first excitation light source 202a and the second excitation light source 202b.
In the doped fiber amplifier of FIG. 54 having the construction just described, excitation light from the first excitation light source 202a and the second excitation light source 202b is split by the coupler 206. Meanwhile, first and second signal light beams are inputted to and excited by, the wave combiners 203a and 203b and then amplified by and outputted from the first and second doped fibers 204a and 204b.
Accordingly, even when only the first excitation light source 202a is used, excitation light is outputted to the first and second wave combiners 203a and 203b by way of the coupler 206 to amplify signal light of the ascending line and the descending line, respectively. Meanwhile, the second excitation light source 202b is employed in order to provide redundancy.
FIG. 55 shows another improved doped fiber amplifier. Referring to FIG. 55, the improved doped fiber amplifier shown has a rearward excitation construction and has a similar construction to that of the doped fiber amplifier described hereinabove with reference to FIG. 54 except that excitation light is inputted from the rear of each of the first and second doped fibers 204a and 204b to excite signal light.
FIG. 56 shows a further improved doped fiber amplifier. Referring to FIG. 56, the improved doped fiber amplifier shown has a bidirectional excitation construction and includes first and second excitation light sources 222a and 222b, a first coupler 226a for inputting excitation light from the first excitation light source 222a and a second coupler 226b for inputting excitation light from the second excitation light source 222b. The doped fiber amplifier further includes, for an ascending line, a first wave combiner 223aprovided on the input side of the first doped fiber 204a and a second second wave combiner 223b provided on the output side of the doped fiber 204a, and for a descending line, a third wave combiner 223c provided on the output side of the doped fiber 204b and a fourth wave combiner 223d provided on the input side of the doped fiber 204b.
In the doped fiber amplifier of the construction just described, the excitation light sources are used commonly for the ascending line and the descending line to amplify signal light.
By the way, in such optical communication systems as described above, the magnitude and the variation of the value of the signal to noise ratio (SNR) which depends upon a signal amplitude, signal light--ASE (accumulated spontaneous emission light) beat noise and inter-ASE beat noise make causes of deterioration of the code error rate in communication.
As seen, for example, from FIG. 57, the signal to noise ratio presents its maximum value when signal light is in a polarization condition in which a maximum transmission line gain is obtained, and the signal to noise ratio presents its minimum value when the signal light is in another polarization condition in which a minimum transmission line gain is obtained. Since the polarization condition of signal light normally varies in response to physical conditions of a transmission line (for example, the temperature, the stress or some other parameter of the optical fiber), the value of the signal to noise ratio temporally varies from a minimum value in a worst polarization condition to a maximum value in a best polarization condition around a certain value.
Where the transmission line gain for a horizontally polarized light component is represented by .GAMMA.s and the gain for a vertically polarized light component is represented by .GAMMA.p, when the transmission line has a polarization dependency, .GAMMA.s.noteq..GAMMA.p. Ordinary signal light involves some polarization, and the polarization condition of the signal light varies with respect to time.
Accordingly, if, for example, linearly polarized light is inputted to such transmission line, then the signal light power at the output of the transmission line varies depending upon the polarization direction .theta. as seen from FIG. 58. In particular, in the case of .GAMMA.s&gt;.GAMMA.p, the signal power after transmission exhibits its minimum (maximum) value when .theta.=.pi./2 (.theta.=0) and exhibits its maximum value when .theta.=0 (.theta.=.pi./2).
However, as described above, in such popular optical communication systems as described above, signal light as a communication medium normally involves some polarization, and If a component of a transmission line such as, for example, an optical amplifier has some polarization dependency of a loss or a gain, then the ratio between the signal light and the ASE varies at an input of the receiver.
Consequently, also the signal to noise ratio varies as seen from FIG. 57. Accordingly, the popular optical communication systems have a subject to be solved in that also the signal to noise ratio is sometimes deteriorated depending upon the polarization condition so that the code error rate may be deteriorated.
Further, where the number of repeating stages of optical amplifiers is comparatively great, deterioration of the signal to noise ratio is accumulated, and even when the gain per one optical amplifier varies, for example, by only 0.1 dB, where the number of repeating stages of optical fibers is several hundreds or more, also the total gain of the optical fibers varies cumulatively. Consequently, the popular optical communication systems have another subject to be solved in that the level of the signal light power on the reception side varies significantly.
Furthermore, when light wavelength multiplexing or light frequency multiplexing is performed to transmit signal light of a plurality of signals, an interaction occurs between the signal light beams as a result of four light wave mixture which is one of a non-linear optical effect of the optical fiber, resulting in deterioration of the signal to noise ratio.